Parasite VS Vampires

Tyler, an orphan is forced to live with his sister, who is never at home during the day. One day, Tyler followed her to her "work place" He saw go down an ally way in the debts of the night. He let his attention wander else-where, a mysterious girl was sleeping on the bench. He turned to his sister, who vanished from the ally and was no where to be seen. It is now his job to find his sister with some help of a mysterious girl. Will he find his sister? Who is this girl?